Move browser image path logic into ExtensionsClient

Move browser image path logic into ExtensionsClient

Currently, extension_file_util::GetBrowserImagePaths
exists and provides universal logic for identifying
sensitive image paths within an extension: i.e. paths to
images which may need to be transcoded before being handed
to the browser to display in its privileged UI.

This list of images depends entirely on the extensions
embedder, so it makes sense to define it within
ExtensionsClient. That's what this CL does. It establishes
such an interface in ExtensionsClient, and moves the very
Chrome-specific logic out of
extension_file_util::GetBrowserImagePaths and into
ChromeExtensionsClient.

BUG=398671
R=scheib@chromium.org
TBR=shuchen@chromium.org,yoz@chromium.org for mechanical updates

Review URL: https://codereview.chromium.org/719403002

Cr-Commit-Position: refs/heads/master@{#304096}